1. Field of the Invention
The invention relates generally to the real time transmission of drilling information to the surface during the drilling operation of a borehole and more particularly relates to an electromagnetic communication system for providing downhole real time drilling parameters at the surface.
2. Description of the Prior Art
Drilling parameters such as drill torque, weight on the drill bit, ambient pressure, and ambient temperature are valuable to a drill rig operator and a search for a reliable method for obtaining this information has continued since the advent of rotor drilling. A prior art technique involved stopping of the drill string, extracting it from the borehole and lowering an instrumentation package in its place. This technique does not provide real time information and only ambient pressures and temperatures are derived thereby which, however, may not be the pressures or temperatures that exist during the drilling operation.
Prior art attempts to develop a drilling telemetry system for providing dynamic information utilized hardwiring or acoustic transmission to transmit downhole drilling information to the surface. One hardwired system utilizes a continuous electrical cable that is lowered inside the drill pipe. An excess cable length is stored on a double loop take-up assembly inside the drill string which is pulled out as additional joints of drill pipe are added. Though this system eliminates the need for an electrical connection for each length of drill pipe, serious problems exist in storing the excess cable length in the drill pipe. Another hardwired system embeds the electrical cable in the walls of the drill pipe and utilizes special connectors manufactured into the tool joints to provide a means of making electrical connections. This system requires a special string of expensive pipe and high reliability of many electrical conductors for efficient operation. In addition to the hardwired electrical systems, acoustic systems in which acoustical waves are launched downhole to propagate along the drill string to be received at the surface were also considered. These systems, however, must compete with acoustic noise that is generated as a result of the drilling operation and generally must extract a signal from a very low signal-to-noise ratio, thus providing a very low probability of signal reception. What is desired is a telemetry system that exhibits a high probability of signal reception without the utilization of additional cables or special drill pipe sections.